cover_url = 'https://elgarblog.files.wordpress.com/2013/10/books1.jpg'
if (!file.exists(cover_file <- 'cover.jpg'))
download.file(cover_url, cover_file, mode = 'wb')
knitr::include_graphics(if (identical(knitr:::pandoc_to(), 'html')) cover_url else cover_file)